A wide range of permselective membranes for gas separation is now known in the art, and commercial gas-separation membranes are beginning to challenge conventional technology in such areas as the production of oxygen-enriched air, nitrogen production for blanketing and other industrial applications, separation of carbon dioxide from methane, and hydrogen recovery from various gas mixtures.
The principal current types of high-performance gas-separation membranes have developed from the anisotropic cellulose acetate reverse-osmosis membranes of Loeb and Sourirajan. (S. Loeb and S. Sourirajan, "Sea Water Demineralization by Means of an Osmotic Membrane", ACS Advances in Chemistry Series 38, 117 (1963)) It is possible to make membranes with good characteristics in this way, and gas-separation membranes of this type have found some commercial application. However, the number of polymers that can be used to make good anisotropic membranes with high gas fluxes and selectivities is limited.
An alternative approach is to coat a Loeb-Sourirajan anisotropic membrane with a thin, permeable sealing layer as diclosed in U.S. Pat. No. 4,230,463 to Henis and Tripodi. The sealing layer, typically silicone rubber, does not function as a selective barrier, but rather serves to plug defects in the permselective membrane, and reduce to negligible amounts the gas flow through these defects. Because the selective layer no longer has to be completely defect-free, membranes of this type can be made thin more easily than regular Loeb-Sourirajan membranes. The increase in flux that is possible with a very thin permselective layer more than compensates for the slight loss in flux caused by the presence of the sealing layer. The selective layer must still be sufficiently thick to withstand normal operating pressures in use.
A third type of gas separation membrane is a composite structure in which the permselective membrane is coated directly onto a high permeability porous support membrane. In contrast to Loeb-Sourirajan membranes, the strength function is separated from the permselective function in this case. U.S. Pat. No. 4,243,701 to Riley and Grabowsky describes such membranes, as does a paper by Ward et al. (W. J. Ward III, W. R. Browall and R. M. Salemme, "Ultrathin Silicone/Polycarbonate Membranes for Gas Separation Processes", J. Memb. Sci. 1, 99 (1976). A disadvantage encountered with these membranes is that the permselective layer must be comparatively thick if a defect-free coating on the microporous support is to be achieved.
Another possible membrane structure is a three-layer composite in which strength, sealing and permselective functions are all separated and performed by different elements of the composite. The membrane substrate layer is a finely microporous support film that has no permselective properties but gives mechanical strength to the composite system. This substrate is coated with a thin rubbery sealing layer which plugs the support defects and provides a smooth surface onto which the top layer may be coated. With this configuration, the permselective layer may be extremely thin, and the resulting composite membrane can produce high permeate fluxes at modest pressures. U.S. Pat. No. 3,874,986 to Browall and Salemme discloses a membrane of this type with a permselective layer of polyphenylene oxide. Japanese Laid-Open Application 59-59214 describes another such membrane in which the permselective layer is polymethylpentene, and Japanese Laid-Open Application 59-112802 gives an example of this type of composite with polybutadiene permselective coating. It is also known in the art to apply yet another permeable coating on top of the permselective layer to protect it from physical damage, and composites of this type are disclosed for example in Japanese Laid-Open Applications 59-66308 and 60-137418. The general concept of coating a composite membrane with a rubbery top layer is disclosed in U.S. Pat. No. 3,980,456 to Browall. Multilayer composite membranes as described above give good results, but are more complex and costly to manufacture than simpler structures. It may be possible to design membrane configurations that give excellent results in small-scale test stamps, but are very difficult to produce in large sheets or rolls suitable for commercial use.
The teachings of the art also include diverse methods for making permselective membranes and membrane elements. Asymmetric Loeb-Sourirajan membranes are normally made by a phase-inversion casting process. Sealing or selective layers may be coated on a microporous support by solvent evaporation. U.S. Pat. No. 4,243,701 to Riley and Grabowski, for example, teaches a method of casting a thin permselective film on the surface of a porous support membrane by a solvent casting technique using halogenated hydrocarbon solvents. Alternatively, films as thin as 50 Angstroms may be prepared by spreading and stretching a polymer solution on water. References describing this liquid casting method include U.S. Pat. No. 3,767,737 to Lundstrom and U.S. Pat. No. 4,132,824 to Kimura et al. The films thus made may be picked up on or laminated to a microporous support by vacuum pick-up or other techniques known in the art.
Alternatively, a selective membrane may be formed in situ on the support membrane by interfacial polymerization. This method has found favor in the preparation of reverse osmosis membranes in particular, as disclosed for instance in U.S. Pat. Nos. 4,277,344 to Cadotte or 4,599,139 to Uemura and Kurihara. Plasma polymerization is yet another technique that may be applied to membrane preparation; U.S. Pat. No. 4,581,043 to van der Scheer covers a method of making gas separation membranes where the ultrathin selective layer is deposited by plasma polymerization.
In spite of the considerable research effort expended on the development of permselective membranes in recent years, the technology is still young, and considerable improvement in membrane performance is necessary before commercialization on a large scale will be possible. A significant problem encountered in making permselective membranes is that improved selectivities for one gas over another are generally obtained at the expense of permeability. Permeability is a measure of the rate at which a particular gas moves through a membrane of standard thickness under a standard pressure difference. In general, polymers that have a high permeability for a gas are not very selective for that gas over others, and highly selective materials are not very permeable. Consequently membrane materials that can give better separation performance than current membranes are needed. To be useful in the art, a polymer must not only have good intrinsic gas permeability and selectivity, but must also have suitable mechanical, thermal and chemical properties to enable it to be processed as a component of a composite membrane, and to withstand the normal operating conditions of that membrane. New polymeric materials with good intrinsic permeability and selectivity properties must be sought. Even when such a polymer is found, however, it may not possess the other necessary characteristics for a membrane material, or the technology required to utilize it may be lacking.
In general, glassy polymers have higher intrinsic selectivities and lower permeabilities than rubbery polymers. Thus to obtain a permselective membrane, made from a glassy material, that has a sufficiently high permeant flux to be of practical use in gas separation applications, it is necessary to make the permselective layer very thin, typically less than 1 micron in thickness. However, it is extremely difficult to coat such thin films on a microporous substrate without forming pinholes or other defects that ruin the selectivity. Thus the various approaches described above, involving the use of rubbery layers above or below the permselective layer, have arisen. There remains a need in the art for materials and techniques that can provide simply made, high-performance, gas separation membranes, without the use of additional sealing or protective layers. The present invention addresses this need.